Lithium ion secondary batteries having a high energy density are extensively used in recent years as power sources for small portable electronic appliances such as cell phones and notebook type personal computers. Such a lithium secondary battery is produced through the steps of stacking or winding sheet-form positive and negative electrodes together with, for example, a porous polyolefin resin film, introducing the resultant stack into a battery container constituted of, for example, a metallic can, subsequently pouring an electrolyte solution into the battery container, and tightly sealing the opening of the battery container.
Recently, however, such small portable electronic appliances are exceedingly strongly desired to be further reduced in size and weight. Under these circumstances, lithium ion secondary batteries also are desired to be further reduced in thickness and weight. Battery containers of the laminated-film type have also come to be used in place of conventional metallic can cases.
Compared with the conventional metallic can cases, battery containers of the laminated-film type have a drawback that an areal pressure for maintaining electrical connection between the separator and each electrode cannot be sufficiently applied to electrode surfaces. Because of this, these battery containers have a problem that the distance between the electrodes partly increases with the lapse of time due to the expansion/contraction of the electrode active materials during battery charge/discharge, resulting in an increase in the internal resistance of the battery and hence in a decrease in battery characteristics. In addition, there is a problem that unevenness of resistance occurs in the battery and this also reduces battery characteristics.
In the case of producing a sheet-form battery having a large area, there has been a problem that the distance between the electrodes cannot be kept constant and the internal resistance of the battery becomes uneven, making it impossible to obtain sufficient battery characteristics.
In order to overcome such problems, it has been proposed to bond electrodes to a separator with an adhesive resin layer constituted of a mixed phase including an electrolyte-solution phase, a polymer gel layer containing the electrolyte solution, and a solid polymer phase (see, for example, patent document 1). Furthermore, a method has been proposed which includes coating a separator with a binder resin solution containing a poly(vinylidene fluoride) resin as a main component, subsequently stacking electrodes on the coated separator, drying the binder resin solution to form an electrode stack, introducing the electrode stack into a battery container, and then pouring an electrolyte solution into the battery container to obtain a battery in which the separator has been adhered to the electrodes (see, for example, patent document 2).
It has also been proposed to obtain a battery containing electrodes adhered to a separator, by bonding a separator impregnated with an electrolyte solution to positive and negative electrodes with a porous adhesive resin layer to bring the separator into close contact with the electrodes and cause the adhesive resin layer to hold the electrolyte solution in the through-holes thereof (see, for example, patent document 3).
However, those processes have had the following problem. The thickness of the adhesive resin layer must be increased in order to obtain sufficient adhesive force between the separator and each electrode. Because of this and because the amount of the electrolyte solution relative to that of the adhesive resin cannot be increased, the resultant battery has increased internal resistance. Consequently, sufficient cycle characteristics and sufficient high-rate discharge characteristics cannot be obtained.
Furthermore, in the battery in which the separator has been adhered to the electrodes with an adhesive resin as described above, the adhesive strength between the separator and each electrode decreases when the battery is placed in a high-temperature environment. As a result, there is a concern that the separator might thermally contract to cause short-circuiting between the electrodes. In addition, although the adhesive resin in the battery is in the state of being swollen with the electrolyte solution, the adhesive resin layer has high internal resistance because electrolyte ions are less apt to diffuse in the adhesive resin than in the electrolyte solution, and exerts adverse influences on battery characteristics.
On the other hand, with respect to porous substrates for use as battery separators, various production processes have been known hitherto. One known process is to produce a sheet made of, for example, a polyolefin resin and stretch the sheet at a high ratio (see, for example, patent document 4). However, the battery separator constituted of such a porous film obtained through high-ratio stretching has a problem that the separator considerably contracts in high-temperature environments, such as the case where a battery has undergone abnormal heating due to internal short-circuiting, etc., and in some cases, comes not to function as a partition between the electrodes.
Consequently, to reduce the degree of heat shrinkage of battery separators which occurs in such a high-temperature environment is regarded as an important subject for improving the safety of batteries. In this respect, a process for producing a porous film for use as a battery separator in order to inhibit the heat shrinkage of battery separators occurring in high-temperature environments is, for example, known. This process includes melt-kneading ultrahigh-molecular polyethylene together with a plasticizer, extruding the mixture through a die into a sheet form, and then extracting and removing the plasticizer to produce the porous film (see patent document 5). However, this process, in contrast to the method described above, has a problem that the porous film obtained has insufficient strength because this film has not undergone stretching.
Moreover, an attempt is recently being made to heighten the charge voltage of batteries as one measure in increasing the capacity of batteries. However, to thus heighten the charge voltage, on the other hand, poses a problem that a large amount of lithium is deintercalated from composite oxides of lithium and cobalt or nickel, which are generally used as positive-electrode active materials, to bring these composite oxides into a higher degree of oxidized state having higher reactivity. As a result, the separator, in particular, deteriorates considerably, resulting in battery performance deterioration.
In order to overcome such a problem, it has been proposed to form a porous layer of a fluororesin such as a polytetrafluoroethylene resin between a separator and a positive electrode (see patent document 6). For example, there is a statement therein to the effect that a preferred method for forming a porous polytetrafluoroethylene resin layer is to spray a suspension of a polytetrafluoroethylene resin on a separator and dry the suspension. However, the layer obtained using this method has an increased thickness to sacrifice battery capacity although rich in porosity. In addition, use of this separator necessitates a large amount of an electrolyte solution.
Patent Document 1: JP-A-09-161814
Patent Document 2: JP-A-11-329439
Patent Document 3: JP-A-10-172606
Patent Document 4: JP-A-09-012756
Patent Document 5: JP-A-05-310989
Patent Document 6: JP-A-2007-157459